Optical measuring apparatus for measuring objects on machines

ABSTRACT

The present invention teaches a method and apparatus for making measurement of an object on a machine, such as a machine tool, using an optical measuring apparatus which includes a light source for generating a beam of light which is incident on a detector. A detection signal is generated within the detector each time the beam is interrupted. The duration and/or frequency of the detection signals are evaluated and an output signal is emitted from the detection only if a further detection signal is present within the detector in a specified time interval from the generation of an earlier detection signal.

[0001] The present invention relates to an optical measuring apparatuswhich enables a coordinate positioning machine (such as a machine tool)to determine the position of an object relative to a reference point. Itmay, for example, be employed on a machine tool for toolsettingoperations.

[0002] A known tool setting device for use on a machine tool includes alight source which generates a fine beam of light which is incident upona detector. During a toolsetting operation, the machine is operated tomove the tool in a direction transverse to the direction of propagationof the light beam until a part of the tool interrupts passage of thelight beam. Detection of this interruption is used to produce a triggersignal in the detecting unit, which is used by the machine to establishthe relative position of its moving parts in order to determinedimensions of the tool. Such devices are known, for example, from GermanPatent Nos. DE 42 385 04 and DE 42 448 69, French Patent No. 2,343,555,European Patent No. 98,930 and U.S. Pat. No. 4,518,257. The devices maybe used additionally for measuring the length or diameter of a tool tomonitor tool breakage or wear.

[0003] The devices disclosed in the above-mentioned patentspecifications use a narrow light beam into or through which the tool ispassed. The detection units detect when the tool breaks into the beamfrom the resulting drop in the intensity of the light falling on them.The trigger signal may be produced as a result of a predetermined dropin the intensity of light falling on the detector as the tool enters thebeam.

[0004] A problem which arises with such optical measuring apparatus isthat coolant used on the machine can drip through the beam, or be thrownoff the rotating tool into the beam, during the measuring operation andgive rise to false trigger signals.

[0005] One method of overcoming this problem which is currently used, isto program the software in the machine controller to perform severalmeasurements until a pre-selected number of measurements falling withina given tolerance have been obtained. The position of the tool is thenassumed to be the average of these measurements. This method can giverise to an unacceptable increase in the measurement cycle time if asignificant number of repeat measurements have to be taken.

[0006] The present invention seeks to alleviate this problem byproviding a method of measurement which can differentiate between agenuine tool detection signal, and a signal produced by a coolant drip.

[0007] According to one aspect of the present invention there isprovided a method of making measurements of an object on a machine toolusing an optical measuring apparatus which includes a light source whichgenerates a beam of light which is incident upon a detector, the methodcomprising the steps of:

[0008] generating a detection signal within the detector each time thebeam is interrupted;

[0009] evaluating the frequency and/or duration of the occurrences ofsaid detection signals;

[0010] emitting an output signal from the detector only if a furtherdetection signal is also present within the detector in a specified timeinterval from the generation of an earlier detection signal.

[0011] The timing of the detection signals can be achieved in variousways.

[0012] In one embodiment of the invention the tool is rotated,preferably, at a known specific speed. This gives rise to the generationof a regular sequence of said signals within the detector as the cuttingedge (or edges) of the tool interrupt the beam. The generation of thefirst one of the said signals is used to initiate a timing sequencewithin the detector which sets a time interval (t₁) substantially equalto the time taken for one revolution of the tool, followed by a secondtime interval (t₂) which is substantially shorter than (t₁) If it is thetool which has generated the signals in the detector then a secondsignal will be generated in the time interval (t₂) as the cutting edgeof the tool comes round again, and if this happens, the detector emitsthe output signal.

[0013] Alternatively, the generation of a detection signal within thedetector can be used to start a clock which emits pulses of shortduration synchronised with the speed of rotation of the tool. Again, ifa second detection signal is generated within the detector during such apulse then the detector emits an output signal. A number of clocks maybe used which start sequentially as the detector generates its detectionsignals, and each of which stops if no second detection signal isgenerated within the detector during the next one of its pulses.

[0014] The invention also includes optical measuring apparatus forcarrying out the method which comprises a light source for generating alight beam and a detector for receiving said beam and which generates asignal when the beam is interrupted., wherein the detector includes adetection circuit which generates a detection signal each time the beamis interrupted, and signal processing means for evaluating the frequencyand/or duration of the occurrences of said detection signals and whichemits an output signal only if a second signal is generated by thedetection circuit within a specified time interval after the occurrenceof an earlier detection signal.

[0015] Examples of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

[0016]FIG. 1 is a perspective view of an optical measuring apparatusincorporating the present invention;

[0017]FIG. 2 is a representation of the output of the detector of FIG.1; and

[0018]FIG. 3 is a block diagram representing the basic elements of theapparatus;

[0019]FIG. 4 illustrates in lines 4 a to 4 f the signals generated invarious parts of the signal processing circuit of the detector.

[0020] Referring now to FIG. 1, the optical measuring apparatus is shownin a set up arranged to operate as a toolsetting apparatus, suitable foruse, for example, on a machine tool. The apparatus includes a lightemitting unit 10 which emits a beam 12 of light, and a light detectingunit 14, where the light beam 12 is detected. Power and signal controlcables to the light emitting and detecting units 10, 14 are routed viainlet ports 16, and both the units 10, 14 are advantageously mounted,via pillars 18, on the base of the machine, either via an intermediatebase 20, to which they are both mounted, or directly to the base of themachine upon which they are to be employed.

[0021] In operation, the apparatus is used for toolsetting by operatingthe machine on which the apparatus is mounted to move the tool in adirection transverse to the direction in which the beam 12 ispropagating. When a predetermined level of occlusion of the beam hasbeen established, the detecting unit 14 emits a trigger signal which isused by the machine to determine the relative position of its relativelymovable parts, thereby to enable dimensions of the tool to bedetermined.

[0022] Further mechanical and optical details of an example of such anapparatus are described in our European Patent Application No.00303749.6 (which is hereby incorporated into this specification byreference) and are not therefore reproduced here.

[0023]FIG. 2 shows the output of the detector in various circumstances.The detector output goes high (i.e. produces a detection signal) asshown by a voltage pulse when the beam is occluded to said predeterminedextent. As can be seen by the first pulse S1 at the left-hand side ofthe drawing this can occur when a coolant drip passes through the beam.

[0024] In the first instance however, the coolant drip is a singleoccurrence, which produces a single short duration pulse.

[0025] When the edge of the rotating cutting tool breaks the beam thereis also a short duration pulse S2, but this is followed by furtherpulses S3 (only one shown) as the same cutting edge comes into the beamagain, or as other cutting edges of a multi-edged tool cut the beam inturn.

[0026] In order to be able to identify the difference between occlusionof the beam by a drip, and occlusion of the beam by the edge of thecutting tool for the first time (which is the event which is required tobe detected to measure the position of the cutting edge), the inventionprovides that a timer in the detector sets a first time interval t₁simultaneously with the detector generating its detection signal. In aspecific embodiment the time interval t₁ is arranged to be equal to thelength of time it takes for one revolution of the tool. At the end ofthe time interval t₁ the timer sets a shorter time interval t₂.

[0027] The detector monitors the time interval t₂ for a second detectionsignal which is unlikely to occur if the first signal is a drip. Thiscan be detected by either a high state of the output signal or a risingedge. If the second detection signal is present the detector issues a“skip” or trigger signal at the end of time interval t₂.

[0028] Since the time intervals t₁ and t₂ are accurately known theinstant of the occurrence of the first rising edge of the detectoroutput due to a cutting edge of the tool obscuring the beam can becalculated.

[0029] In order to calculate the timings, the speed of rotation of themachine spindle, and hence the cutting tool must be set. In order tokeep the time for the measuring operation down a reasonable level, thespindle speed was set during experimentation at 1000 rpm so that t₁ wasostensibly 60 ms. The interval t₂ however, has to be large enough tocater for slight variations in spindle speed for example up to 5%, whichwould cause a 3 ms variation in t₁.

[0030] To centre the trigger signal in the time interval t₂ t₂ is set att₁+½t which must be equal to 60 ms. Thus t₁ was actually set at 58.5 ms.

[0031] It is not necessary for the speed of rotation of the tool to beknown prior to measurements being taken since it can be measured bytiming the distance between the rising edges of the first twoconsecutive pulses out of the sequence of pulses generated by thedetector. The time interval (t₁) can then be set between the second andthird rising edges and (t₂) can be timed from the third rising edge.

[0032] The basic elements of the apparatus of the invention are shown inblock diagram form in FIG. 3. The light beam 12 from the transmitter 10entering the detector 14 impinges on a photodetector in a detectioncircuit 72 which produces the signals when the beam is interrupted.Signals generated in the detection circuit 72 are passed to a signalprocessor 74 which includes the necessary timing devices for signalanalysis. The detector output signals are passed directly to the machinecontroller 80 which stops the machine, and evaluates the machine scalereadings to determine the position of the machine.

[0033]FIG. 4 illustrates an alternative embodiment in which one or moreclocks in the detector are used, each of which generates a series ofpulses initiated by the detector issuing a signal to indicate that thebeam has been interrupted;

[0034]FIG. 4a shows an example of a series of events created by amixture of teeth and drips interrupting the beam;

[0035]FIG. 4b shows the signals created by these events converted topulses at the output of a comparator in the signal processor of thedetector;

[0036]FIG. 4c shows the situation which occurs in the detector when asingle clock is used;

[0037]FIG. 4d shows the situation which occurs in the detector when asecond clock is used;

[0038]FIGS. 4e and 4 f respectively show the results of combining thecomparator output with the outputs of clock 1 and clock 2.

[0039] It can be seen from FIG. 4a that the drips occur and interruptthe beam at random intervals while the beam interruptions caused by theedge of the tool occur at regular intervals. Each beam interruption islabelled as a numbered event E.

[0040]FIG. 4b shows the comparator output pulses corresponding to theevents.

[0041]FIG. 4c shows that the first clock starts when event E1 occurs butbecause there is no event occurring when the second clock pulse is sentto the signal processor the clock is stopped. The clock is started againon the occurrence of event E3 which is also a drip, but again becauseits second pulse occurs between events E4 and E5 it will not see eventE4 and will stop. The situation is the same when it starts again atevent E5. Only when it starts again at event E7 will its pulses besynchronised with the occurrences of the cutting edge of the toolinterrupting the beam at event E9 and beyond so that a trigger signalwill be produced on event E7. This signal will have missed the firstoccurrence of the cutting edge interrupting the beam at E6 and willproduce an erroneous reading.

[0042] With the embodiment which uses two clocks however, as shown inFIG. 4d, the second clock will be started on event E6 because at thattime the first clock is running. Since event E6 is an interruption ofthe beam by an edge of the cutting tool there will be a further event E7occurring when the clock pulse is generated. The signal processor willrecognise that the clock pulses and the events have now becomesynchronised and will generate a trigger signal on the rising edge ofthe pulse. Since the generation of the clock pulses and the beaminterruptions are synchronised, the time at which the first beaminterruption occurred can easily be determined.

[0043] The situation is less complex if the first event is caused by acutting edge of the tool, because the first clock will start and willsee a synchronised event during its first pulse and cause a triggersignal at that time.

[0044] If a drip occurs between the first and second events this will beignored by the first clock since it will not occur during its firstpulse. Hence the drip will not affect the generation of the triggersignal under these circumstances.

[0045] Although the invention has been described using one or twoclocks, other benefits may be achieved if more clocks were used whichare set at different frequencies.

[0046] For example, the apparatus could be used at different speeds ofspindle rotation without having to re-set the timing of the existingclocks, and additional clocks would allow the apparatus to deal with anatmosphere where a lot of drips could be expected. The number of clocksused would be a trade-off between the benefits to be obtained, and theexpense of the additional signal processing capability required.

[0047] The invention can also be used when the tool is not rotating tomeasure tool length or diameter during tool setting, or for toolbreakage detection. In such an embodiment the tool is moved at rightangles to the beam until its tip or flank interrupts the beam. Thesignal produced by the detector is used to start a clock in the signalprocessor which evaluates the detector output after a time t. If thedetector output is still high at that time, denoting a signal is stillpresent, the signal processor produces a trigger signal.

[0048] In yet another embodiment, the signal processor may incorporate adevice which identifies synchronous events. The inputs to the device aresamples at regular intervals and the samples are stored in a buffer offixed length, the new contents constantly overriding the old. The buffercan be implemented using a shift register which monitors the detectoroutput and writes its current state into the shift register each time asample is taken. If the buffer is split across two bytes, testing forrepetitive patterns can be achieved by comparing the two halves. Forexample, if the sample rate is eight times the speed of rotation of thetool and the result of each sample is shifted through two eight bitregisters, when a first signal occurs this shows up as a high (1) in thefirst cell of the register. The sample moves through the register untilit is passed into the first cell on the second half. If there has beenno synchronous event the sample passes through the register to the end.

[0049] If however, another high sample is received in the first cell ofthe first half of the register just as the first sample moves into thefirst cell of the second half of the register then the two halves willbecome identical once again and a trigger signal will be issued.

[0050] The invention has been described with reference to theelimination of spurious trigger signals in an optical measuringapparatus on a machine tool, but may have wider application using otherforms of optical measuring apparatus on other types of machine. Thescope of the invention is therefore, to be taken as that defined by theappended claims.

1. A method of making measurements of an object on a machine using anoptical measuring apparatus which includes a light source whichgenerates a beam of light which is incident upon a detector, the methodcomprising the steps of: generating a detection signal within thedetector each time the beam is interrupted; evaluating the frequencyand/or duration of the occurrences of said detection signals; emittingan output signal from the detector only if a further detection signal ispresent within the detector in a specified time interval from thegeneration of an earlier detection signal.
 2. A method according toclaim 1 and comprising the further steps of: rotating the object;generating a first time interval (t₁) which is dependent on the speed ofrotation of the object; and generating the specified time interval as atime interval (t₂) which is shorter than (t₁) and commences at the endof the interval (t₁).
 3. A method according to claim 2 wherein theobject is a tool on a machine tool and the tool is rotated at a knownspecific speed and the time interval (t₁) is substantially equal to thetime taken for one revolution of the tool.
 4. A method according toclaim 3 wherein the apparatus further includes a clock, the methodcomprising the further steps of: rotating the tool; causing the clock toinitiate the emission of a series of pulses of short duration which aresynchronised with the speed of rotation of the tool, the first pulsebeing emitted to coincide with a detection signal being generated in thedetector; emitting an output signal from the detector only if adetection signal is also present within the detector during theexistence of a clock pulse; stopping the clock if no such detectionsignal is present in the detector.
 5. A method according to claim 4wherein a clock pulse is produced for each revolution of the tool, andan output signal is emitted from the detector only if a detection signalis present with the detector during the next pulse emitted by the clockfollowing said first pulse.
 6. A method according to claim 4 wherein theapparatus includes two clocks the method comprising the further stepsof: causing a first clock to initiate the emission of a first series ofsaid pulses when a detection signal is generated within the detector;causing the second clock to initiate the emission of a second series ofsaid pulses commencing with the generation of a further detection signalwithin the detector in the interval between two successive pulses of thefirst clock; and emitting an output signal from the detector if adetection signal is also present within the detector during theexistence of the next pulse in the second series if the detector has notemitted an output signal based on the first series of pulses.
 7. Amethod according to claim 6 wherein the apparatus includes additionalclocks, the method comprising the steps of sequentially initiating theemission of respective series of said pulses if a detection signal isgenerated within the detector and all of the previously started clocksare running.
 8. A method according to claim 6 wherein the apparatusincludes additional clocks, the method comprising the steps of settingthe clocks to produce respective series' of pulses at differentfrequencies set to coincide with different speeds of rotation of thetool, and causing the initiation of the emission of a said series ofpulses appropriate to the speed of rotation of the tool.
 9. Opticalapparatus for measuring objects on machines comprising a light sourcefor generating a light beam and a detector for receiving said beam andwhich generates a signal when the beam is interrupted, wherein thedetector includes a detection circuit which generates a signal each timethe beam is interrupted, and signal processing means for evaluating thefrequency and/or duration of the occurrences of said signals and whichemits an output signal only if a second signal is generated by thedetection circuit within a specified time interval after the occurrenceof said signal.
 10. Optical measuring apparatus according to claim 9 andwherein the object is a tool on a machine tool.